1. Field of the Invention
The present invention is directed to a photographic optical system and an image pickup apparatus having the same, which are particularly suitable for a video camera, a digital still camera, a TV camera, a monitoring camera, a silver-halide film camera, and the like.
2. Description of the Related Art
In the related art, as a photographic optical system having a long focal length, there is a known telephoto type of photographic optical system (telephoto lens) including a front lens unit having a positive refractive power and a rear lens unit having a negative refractive power disposed in order from an object side to an image side. In general, regarding a telephoto lens having a long focal length, there is a tendency that chromatic aberration, especially axial chromatic aberration and chromatic aberration of magnification, among various aberrations deteriorates as the focal length is increased.
There is a known telephoto lens of which chromatic aberration is corrected (removed) by a combination of a positive lens formed by using a low-dispersion material having an anomalous partial dispersion property, such as fluorite or FK01 (trade name, manufactured by OHARA INC.) and a negative lens formed by using a high-dispersion material. U.S. Pat. No. 5,757,555 discusses an inner-focus type of telephoto lens having a focal length of about 300 mm and an F-number of about 4.
In addition, in general, with respect to a telephoto lens having a small F-number, there is a tendency that especially spherical aberration and coma among various aberrations deteriorate as the F-number is decreased. As a method of correcting spherical aberration, coma, or the like of such a telephoto lens having a small F-number, there is a method of increasing a degree of freedom with respect to correction of aberration by increasing the number of lens elements. There is a known telephoto lens of which spherical aberration or coma is reduced according to this method. U.S. Pat. No. 5,745,306 discusses a large-aperture-ratio inner-focus type of telephoto lens having a focal length of about 300 mm and an F-number of about 2.8.
On the other hand, as a method of correcting various aberrations of an optical system including chromatic aberration, and of reducing lens weight, there is a method which uses a diffractive optical element in which, on a lens surface or in a portion of the optical system, a diffractive optical portion having a diffraction function provided on a substrate is installed. There is known an optical system having a reduced total lens weight which is achieved by decreasing the total lens length while correcting chromatic aberration in the way described above and by forming the lens using a glass material having a relatively low specific gravity. U.S. Pat. No. 7,426,083 discusses a large-aperture-ratio telephoto lens having a focal length of about 500 mm and an F-number of about 4. In U.S. Pat. No. 7,426,083, the correction of chromatic aberration and the reduction of total weight are contrived by decreasing the total lens length and using a glass material having a relatively low specific gravity for a positive lens of a front lens unit.
In addition, as a method of correcting chromatic aberration of an optical system, there is a method using an optical element made of solid material having a larger anomalous partial dispersion property than a glass material. There is known an optical system of which the total lens weight is reduced by correcting chromatic aberration and decreasing the total lens length according to this method and by configuring the lens using a glass material having a relatively low specific gravity. U.S. Pat. No. 7,193,789 discusses a telephoto type of optical system having a focal length of about 300 mm and an F-number of about 4. In U.S. Pat. No. 7,193,789, the correction of chromatic aberration is achieved by using a solid material having a large anomalous partial dispersion property for an optical element having a positive power in a front lens unit.
In general, in a large number of photographic lenses (optical systems), focusing is performed by moving the entire photographic lens or moving a partial lens unit, a part of the photographic lens. Of the both methods, in the case of a telephoto lens having a long focal length, since the telephoto lens is large in size and heavy, it is difficult to perform focusing by moving the entire telephoto lens in terms of mechanism.
Therefore, in the related art, the telephoto lens performs focusing by moving a partial lens unit of the telephoto lens. The telephoto type of optical system uses an inner focusing system in which focusing is performed by moving a partial lens unit, other than a front lens unit, disposed in a central portion of the optical system which is relatively small and light. All of the above-described telephoto lenses each include a first lens unit having a positive refractive power and a second lens unit having a negative refractive-power that are disposed in order from an object side, and perform focusing by moving a partial lens unit which is a part of the second lens unit along the optical axis.
In the telephoto lenses, to appropriately correct various aberrations including chromatic aberration, it is an effective way to use an optical material having an appropriate anomalous partial dispersion and a low dispersion for a front lens unit having a larger effective diameter and a positive refractive power. In the telephoto lens, when the front lens unit having a positive refractive power has an inappropriate lens configuration, the entire system increases in both size and weight. In addition, with respect to the telephoto lens, since the front lens unit is large and heavy, focusing is not performed by moving the front lens unit. It is fast and effective focusing to use an inner focusing system in which focusing is performed by moving a partial lens unit having a small size and a low weight among lens units instead of the above method.
However, when the inner focusing type is employed in a state where the front lens unit has an inappropriate lens configuration, a variation in aberration, especially, a variation in chromatic aberration increases during focusing, and it is very difficult to compensate for this change.